Storm Forecast
Valid: Tue 02 Oct 2018 06:00 to Wed 03 Oct 2018 06:00 UTC
Issued: Mon 01 Oct 2018 23:10
Forecaster: DAFIS

A level 1 was issued for parts of Denmark, N Germany, Poland, S Sweden, Latvia, and Lithuania mainly for convective severe winds gusts.

A level 1 was issued for parts of Italy, S Croatia, Bosnia and Herzegovina, Montenegro, Albania, Greece, N Algeria and Tunisia mainly for excessive precipitation, severe wind gusts, large hail or large amounts of small hail and tornadoes.

A level 1 was issued for SW Russia mainly for large hail and severe wind gusts.


SYNOPSIS

A positively tilted long-wave trough is affecting a large part of the continent, and while a ridge is intensifying in the western parts towards Central Europe, it will eventually lead to the formation of a cold drop in Central Mediterranean. This cut-off low, with substantial positive vorticity, will destabilize parts of Italy, Corsica, Adriatic Sea, and North Africa. The strong baroclinicity over the Gulf of Lion will progressively weaken during Tuesday as the LL cyclone in the Tyrrhenian Sea is moving south.

In North Europe, a sharp cold front is currently crossing Poland and during the night of Tuesday, a short-wave trough is expected to form over S Scandinavia and move quickly E-SE towards the Baltic States. Another short-wave trough is visible in the analysis maps over the Black Sea, with its vorticity max crossing SW Russia on Tuesday and Wednesday morning.


DISCUSSION

... North Europe ....

A fast-moving short-wave trough with strong QG-forcing will be the reason for widespread instability over the Baltic Sea, where lapse rates will exceed 7 K/km. NWP models forecast some hundreds J/kg of CAPE overlapped with strong DLS. Most of the precipitation will be convective and some areas will get graupel or small amounts of hail. The main threat though will be severe convective wind gusts as the 850 hPa wind field seems vigorous, which can be easily translated to the surface within the storm cells. Limited CAPE is expected over land, so most of the lightning activity should be found over the sea.

.... Central Mediterranean Sea ....

The cut-off low which is about to form over the Gulf of Genoa will provide very low geopotentials over the warm sea, creating some steep lapse rates. The positive vorticity that carries will also provide synoptic lift to already unstable air masses. The east part of the level-1 area is covered by abundant moisture while on the other hand, the PBL in the Tyrrhenian Sea is quite dry for the standards of this time of the year with about 6-8 g/kg mixing ratios, because of the strong Mistral winds the previous days that pushed the moist air masses more to the south. Nevertheless, some storms have been developed in the afternoon of Monday and they will continue to affect Corsica, Sardinia and W Italy during the early morning of Tuesday but with limited potential for severe weather. Later during the day, we expect several storm cells to form especially near the core of the upper-level disturbance.

Regarding moisture, NWP models are more generous for the north coasts of Algeria and Tunisia with mixing ratios exceeding 10 g/kg. In North Africa, the main threat will be excessive precipitation due to training convection. For the other parts of the level-1 area, large hail and severe wind gusts are possible in South Italy where CAPESHEAR is maximized, and steep lapse rates should assist the formation of deep updraughts. In the morning of Wednesday, the interest will be shifted towards the west coasts of the Balkans where also training convection may cause flash floods and some severe wind events.

Waterspouts are well expected under the core of the cut-off low west of Italy, where rotating updraughts will be more easy to form than the surroundings and probabilities are high near the convergence zones, near the coasts.

.... SW Russia ....

A plume of warm and moist and thus unstable air mass is traveling NE over the Black Sea towards SW Russia while a short-wave trough is also moving towards this area. Steep lapse rates create up to 2000 J/kg, overlapping with strong DLS. Some large hail events and severe wind gusts are expected produced by slow-moving storm cells. Storm activity will weaken during the night.